The present invention relates to a rotation detecting apparatus suitable for detecting a rotation speed of an engine or electric motor.
Generally, it is well known that a type of a rotation detecting apparatus for detecting a rotation speed of an engine is generally an electromagnetic type comprising a magnet and a coil and of an optical type comprising a light emitting diode and a phototransistor. Japanese Utility Model Provisional Publication No. 3-91969 discloses an electromagnetic type rotation detecting apparatus where a Hall-effect device is used as a magnetic detecting element.
Herein, exemplifying a case that a conventional electromagnetic type rotation detecting apparatus 3 is used for detecting a rotation speed of an engine, the conventional rotation detecting apparatus will be explained on the basis of FIG. 4.
A rotation plate 1 functioning as a rotation member is formed into a disc with teeth from magnetic material. The rotation plate 1 is, for example, arranged to be fixed to a crankshaft (not shown) of an engine and to be rotated with the crankshaft in the direction indicated by the arrow A of FIG. 4. A plurality of tooth portions 2 are protruded from an outer peripheral portion of the rotation plate 1 into diametrical directions of the rotation plate 1. The tooth portions 2 are arranged at predetermined intervals in the circumferential direction. The electromagnetic type rotation detecting apparatus 3 comprises a casing 4 which is constituted by a cylindrical body 5, an installation bracket portion 6 and a L-shaped connector portion 7. The cylindrical body 5 has a bottom portion 5A. The installation bracket portion 6 is integrally formed with the cylindrical portion 5 so as to protrude from an upper end side of the cylindrical body 5 in the circumferential direction while providing an installation hole portion 6A. The L-shaped connector portion 7 is press-fitted with an opening portion of the cylindrical portion 5 to cover the opening portion. The casing 4 is formed into a L-shape from resin material. The installation bracket portion 6 is fixed to a frame of the engine by means of bolts and nuts (not shown) so as to locate the bottom portion 5A of the cylindrical member 5 in the vicinity of the rotation plate 1. An O-ring 8 is disposed between the installation bracket portion 6 and the connector portion 7. A magnet 9 is formed into a cylindrical shape, and a core member 10 is disposed in contact with an end portion of the magnet 9 in the axial direction. The core member 10 is made from magnetic material such as iron and ferrite, and is constituted by a large-diameter portion 10A in contact with and a small-diameter portion 10B formed at a tip end of the core member 10. A pair of Hall-effect devices 11A and 11B functioning as a magnetic detecting element are disposed at a tip end portion of the small-diameter portion 10B of the core member 10. The Hall-effect devices 11A and 11B detect a change of magnetic flux density between the magnet 9 and the rotation plate 1. A supporting plate 12 inserted into the cylindrical member 5 is formed into a generally U-shape. Disposed between a lower end of the supporting plate 12 and the bottom portion 5A of the cylindrical member 5 are the magnet 9, the core member 10 and the Hall-effect devices 11A and 11B. Both arms of the supporting portion 12 support both sides of a circuit base plate 13, respectively, so that the circuit base plate 13 is supported by the supporting plate 12 in the cylindrical member 5. A signal processing circuit 14 such as a waveform shaping circuit is constructed on the circuit base plate 13 by means of electronic parts. The Hall-effect devices 11A and 11B are connected to a terminal of the signal processing circuit 14, and the other terminal of the signal processing circuit 14 is connected to a lead wire 15 through which the signal processing circuit 14 is connected to an external microcomputer (not shown).
The operation of this conventional rotation detecting apparatus 3 will be discussed hereinafter.
First, the magnet 9 and the core member 10 produce magnetic field between the rotation detecting apparatus 3 and the rotation plate 1. At every times when each tooth portion 9 of the rotation plate 1 passes through the magnetic field according to the rotation of the rotation plate 1 in the direction indicated by the arrow A of FIG. 4, the magnetic flux density of the magnetic field is changed. The Hall-effect devices 11A and 11B detect the change of the magnetic flux density and output a detection signal based on the change to the signal processing circuit 14.
Next, the signal processing circuit 14 analog-to-digital converts the detection signal and output a pulse signal, in order to eliminate an excess component such as noises in the detection signal outputted from the Hall-effect devices 11A and 11B. The pulse signal is outputted to an external control unit. The external control unit executes the detection of the rotation speed of the engine from the pulse signal, and executes various controls such as a fuel injection control and a control of ignition timing of the engine.
However, since the conventional rotation detecting apparatus 3 is constituted by a casing 4, a magnet 9, a core member 10, a circuit base plate 14 and the supporting plate 12, it causes problems such that the number of the parts is increased, the assembly step is complicated and the production cost becomes high. Further, the casing 4 is constructed by two parts, that is, by the cylindrical member 5 and the connector portion 7, and the connector portion 7 is engaged with and fixed to the opening portion of the cylindrical member 5 while the O-ring 8 is installed therebetween. Furthermore, since the rotation detecting apparatus 3 is disposed in the vicinity of the engine, the casing 4 is thermally expanded due to the heat of the engine. By this thermal expansion, the cylindrical member 5 and the connector portion 7 generate thermal deformations, respectively. This causes a problem that the O-ring 8 installed therebetween can not prevent rain-water and muddy water from invading from outside to inside the apparatus 3.